3.10 Options That Control Optimization

These options control various sorts of optimizations.

Without any optimization option, the compiler's goal is to reduce the
cost of compilation and to make debugging produce the expected
results. Statements are independent: if you stop the program with a
breakpoint between statements, you can then assign a new value to any
variable or change the program counter to any other statement in the
function and get exactly the results you would expect from the source
code.

Turning on optimization flags makes the compiler attempt to improve
the performance and/or code size at the expense of compilation time
and possibly the ability to debug the program.

The compiler performs optimization based on the knowledge it has of the
program. Compiling multiple files at once to a single output file mode allows
the compiler to use information gained from all of the files when compiling
each of them.

Not all optimizations are controlled directly by a flag. Only
optimizations that have a flag are listed in this section.

Most optimizations are only enabled if an -O level is set on
the command line. Otherwise they are disabled, even if individual
optimization flags are specified.

Depending on the target and how GCC was configured, a slightly different
set of optimizations may be enabled at each -O level than
those listed here. You can invoke GCC with `-Q --help=optimizers'
to find out the exact set of optimizations that are enabled at each level.
See Overall Options, for examples.

-O

-O1

Optimize. Optimizing compilation takes somewhat more time, and a lot
more memory for a large function.

With -O, the compiler tries to reduce code size and execution
time, without performing any optimizations that take a great deal of
compilation time.

-O also turns on -fomit-frame-pointer on machines
where doing so does not interfere with debugging.

-O2

Optimize even more. GCC performs nearly all supported optimizations
that do not involve a space-speed tradeoff.
As compared to -O, this option increases both compilation time
and the performance of the generated code.

-O2 turns on all optimization flags specified by -O. It
also turns on the following optimization flags:

Please note the warning under -fgcse about
invoking -O2 on programs that use computed gotos.

-O3

Optimize yet more. -O3 turns on all optimizations specified
by -O2 and also turns on the -finline-functions,
-funswitch-loops, -fpredictive-commoning,
-fgcse-after-reload, -ftree-vectorize and
-fipa-cp-clone options.

-O0

Reduce compilation time and make debugging produce the expected
results. This is the default.

-Os

Optimize for size. -Os enables all -O2 optimizations that
do not typically increase code size. It also performs further
optimizations designed to reduce code size.

Disregard strict standards compliance. -Ofast enables all
-O3 optimizations. It also enables optimizations that are not
valid for all standard compliant programs.
It turns on -ffast-math.

If you use multiple -O options, with or without level numbers,
the last such option is the one that is effective.

Options of the form -fflag specify machine-independent
flags. Most flags have both positive and negative forms; the negative
form of -ffoo would be -fno-foo. In the table
below, only one of the forms is listed—the one you typically will
use. You can figure out the other form by either removing `no-'
or adding it.

The following options control specific optimizations. They are either
activated by -O options or are related to ones that are. You
can use the following flags in the rare cases when “fine-tuning” of
optimizations to be performed is desired.

-fno-default-inline

Do not make member functions inline by default merely because they are
defined inside the class scope (C++ only). Otherwise, when you specify
-O, member functions defined inside class scope are compiled
inline by default; i.e., you don't need to add `inline' in front of
the member function name.

-fno-defer-pop

Always pop the arguments to each function call as soon as that function
returns. For machines which must pop arguments after a function call,
the compiler normally lets arguments accumulate on the stack for several
function calls and pops them all at once.

Disabled at levels -O, -O2, -O3, -Os.

-fforward-propagate

Perform a forward propagation pass on RTL. The pass tries to combine two
instructions and checks if the result can be simplified. If loop unrolling
is active, two passes are performed and the second is scheduled after
loop unrolling.

This option is enabled by default at optimization levels -O,
-O2, -O3, -Os.

-ffp-contract=style

-ffp-contract=off disables floating-point expression contraction.
-ffp-contract=fast enables floating-point expression contraction
such as forming of fused multiply-add operations if the target has
native support for them.
-ffp-contract=on enables floating-point expression contraction
if allowed by the language standard. This is currently not implemented
and treated equal to -ffp-contract=off.

The default is -ffp-contract=fast.

-fomit-frame-pointer

Don't keep the frame pointer in a register for functions that
don't need one. This avoids the instructions to save, set up and
restore frame pointers; it also makes an extra register available
in many functions. It also makes debugging impossible on
some machines.

On some machines, such as the VAX, this flag has no effect, because
the standard calling sequence automatically handles the frame pointer
and nothing is saved by pretending it doesn't exist. The
machine-description macro FRAME_POINTER_REQUIRED controls
whether a target machine supports this flag. See Register Usage.

Starting with GCC version 4.6, the default setting (when not optimizing for
size) for 32-bit Linux x86 and 32-bit Darwin x86 targets has been changed to
-fomit-frame-pointer. The default can be reverted to
-fno-omit-frame-pointer by configuring GCC with the
--enable-frame-pointer configure option.

Enabled at levels -O, -O2, -O3, -Os.

-foptimize-sibling-calls

Optimize sibling and tail recursive calls.

Enabled at levels -O2, -O3, -Os.

-fno-inline

Don't pay attention to the inline keyword. Normally this option
is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded inline.

-finline-small-functions

Integrate functions into their callers when their body is smaller than expected
function call code (so overall size of program gets smaller). The compiler
heuristically decides which functions are simple enough to be worth integrating
in this way.

Enabled at level -O2.

-findirect-inlining

Inline also indirect calls that are discovered to be known at compile
time thanks to previous inlining. This option has any effect only
when inlining itself is turned on by the -finline-functions
or -finline-small-functions options.

Enabled at level -O2.

-finline-functions

Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be worth
integrating in this way.

If all calls to a given function are integrated, and the function is
declared static, then the function is normally not output as
assembler code in its own right.

Enabled at level -O3.

-finline-functions-called-once

Consider all static functions called once for inlining into their
caller even if they are not marked inline. If a call to a given
function is integrated, then the function is not output as assembler code
in its own right.

Enabled at levels -O1, -O2, -O3 and -Os.

-fearly-inlining

Inline functions marked by always_inline and functions whose body seems
smaller than the function call overhead early before doing
-fprofile-generate instrumentation and real inlining pass. Doing so
makes profiling significantly cheaper and usually inlining faster on programs
having large chains of nested wrapper functions.

Enabled by default.

-fipa-sra

Perform interprocedural scalar replacement of aggregates, removal of
unused parameters and replacement of parameters passed by reference
by parameters passed by value.

Enabled at levels -O2, -O3 and -Os.

-finline-limit=n

By default, GCC limits the size of functions that can be inlined. This flag
allows coarse control of this limit. n is the size of functions that
can be inlined in number of pseudo instructions.

Inlining is actually controlled by a number of parameters, which may be
specified individually by using --param name=value.
The -finline-limit=n option sets some of these parameters
as follows:

max-inline-insns-single

is set to n/2.

max-inline-insns-auto

is set to n/2.

See below for a documentation of the individual
parameters controlling inlining and for the defaults of these parameters.

Note: there may be no value to -finline-limit that results
in default behavior.

Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way does it represent a count
of assembly instructions and as such its exact meaning might change from one
release to an another.

-fno-keep-inline-dllexport

This is a more fine-grained version of -fkeep-inline-functions,
which applies only to functions that are declared using the dllexport
attribute or declspec (See Declaring Attributes of Functions.)

-fkeep-inline-functions

In C, emit static functions that are declared inline
into the object file, even if the function has been inlined into all
of its callers. This switch does not affect functions using the
extern inline extension in GNU C90. In C++, emit any and all
inline functions into the object file.

GCC enables this option by default. If you want to force the compiler to
check if the variable was referenced, regardless of whether or not
optimization is turned on, use the -fno-keep-static-consts option.

This option is the default for optimized compilation if the assembler and
linker support it. Use -fno-merge-constants to inhibit this
behavior.

Enabled at levels -O, -O2, -O3, -Os.

-fmerge-all-constants

Attempt to merge identical constants and identical variables.

This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating point
types. Languages like C or C++ require each variable, including multiple
instances of the same variable in recursive calls, to have distinct locations,
so using this option will result in non-conforming
behavior.

-fmodulo-sched

Perform swing modulo scheduling immediately before the first scheduling
pass. This pass looks at innermost loops and reorders their
instructions by overlapping different iterations.

-fmodulo-sched-allow-regmoves

Perform more aggressive SMS based modulo scheduling with register moves
allowed. By setting this flag certain anti-dependences edges will be
deleted which will trigger the generation of reg-moves based on the
life-range analysis. This option is effective only with
-fmodulo-sched enabled.

-fno-branch-count-reg

Do not use “decrement and branch” instructions on a count register,
but instead generate a sequence of instructions that decrement a
register, compare it against zero, then branch based upon the result.
This option is only meaningful on architectures that support such
instructions, which include x86, PowerPC, IA-64 and S/390.

The default is -fbranch-count-reg.

-fno-function-cse

Do not put function addresses in registers; make each instruction that
calls a constant function contain the function's address explicitly.

This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the optimizations
performed when this option is not used.

The default is -ffunction-cse

-fno-zero-initialized-in-bss

If the target supports a BSS section, GCC by default puts variables that
are initialized to zero into BSS. This can save space in the resulting
code.

This option turns off this behavior because some programs explicitly
rely on variables going to the data section. E.g., so that the
resulting executable can find the beginning of that section and/or make
assumptions based on that.

The default is -fzero-initialized-in-bss.

-fmudflap -fmudflapth -fmudflapir

For front-ends that support it (C and C++), instrument all risky
pointer/array dereferencing operations, some standard library
string/heap functions, and some other associated constructs with
range/validity tests. Modules so instrumented should be immune to
buffer overflows, invalid heap use, and some other classes of C/C++
programming errors. The instrumentation relies on a separate runtime
library (libmudflap), which will be linked into a program if
-fmudflap is given at link time. Run-time behavior of the
instrumented program is controlled by the MUDFLAP_OPTIONS
environment variable. See env MUDFLAP_OPTIONS=-help a.out
for its options.

Use -fmudflapth instead of -fmudflap to compile and to
link if your program is multi-threaded. Use -fmudflapir, in
addition to -fmudflap or -fmudflapth, if
instrumentation should ignore pointer reads. This produces less
instrumentation (and therefore faster execution) and still provides
some protection against outright memory corrupting writes, but allows
erroneously read data to propagate within a program.

-fthread-jumps

Perform optimizations where we check to see if a jump branches to a
location where another comparison subsumed by the first is found. If
so, the first branch is redirected to either the destination of the
second branch or a point immediately following it, depending on whether
the condition is known to be true or false.

Enabled at levels -O2, -O3, -Os.

-fsplit-wide-types

When using a type that occupies multiple registers, such as long
long on a 32-bit system, split the registers apart and allocate them
independently. This normally generates better code for those types,
but may make debugging more difficult.

Enabled at levels -O, -O2, -O3,
-Os.

-fcse-follow-jumps

In common subexpression elimination (CSE), scan through jump instructions
when the target of the jump is not reached by any other path. For
example, when CSE encounters an if statement with an
else clause, CSE will follow the jump when the condition
tested is false.

Enabled at levels -O2, -O3, -Os.

-fcse-skip-blocks

This is similar to -fcse-follow-jumps, but causes CSE to
follow jumps which conditionally skip over blocks. When CSE
encounters a simple if statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the
body of the if.

Enabled at levels -O2, -O3, -Os.

-frerun-cse-after-loop

Re-run common subexpression elimination after loop optimizations has been
performed.

Enabled at levels -O2, -O3, -Os.

-fgcse

Perform a global common subexpression elimination pass.
This pass also performs global constant and copy propagation.

Note: When compiling a program using computed gotos, a GCC
extension, you may get better runtime performance if you disable
the global common subexpression elimination pass by adding
-fno-gcse to the command line.

Enabled at levels -O2, -O3, -Os.

-fgcse-lm

When -fgcse-lm is enabled, global common subexpression elimination will
attempt to move loads which are only killed by stores into themselves. This
allows a loop containing a load/store sequence to be changed to a load outside
the loop, and a copy/store within the loop.

Enabled by default when gcse is enabled.

-fgcse-sm

When -fgcse-sm is enabled, a store motion pass is run after
global common subexpression elimination. This pass will attempt to move
stores out of loops. When used in conjunction with -fgcse-lm,
loops containing a load/store sequence can be changed to a load before
the loop and a store after the loop.

Not enabled at any optimization level.

-fgcse-las

When -fgcse-las is enabled, the global common subexpression
elimination pass eliminates redundant loads that come after stores to the
same memory location (both partial and full redundancies).

Not enabled at any optimization level.

-fgcse-after-reload

When -fgcse-after-reload is enabled, a redundant load elimination
pass is performed after reload. The purpose of this pass is to cleanup
redundant spilling.

-funsafe-loop-optimizations

If given, the loop optimizer will assume that loop indices do not
overflow, and that the loops with nontrivial exit condition are not
infinite. This enables a wider range of loop optimizations even if
the loop optimizer itself cannot prove that these assumptions are valid.
Using -Wunsafe-loop-optimizations, the compiler will warn you
if it finds this kind of loop.

-fcrossjumping

Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
resulting code may or may not perform better than without cross-jumping.

Enabled at levels -O2, -O3, -Os.

-fauto-inc-dec

Combine increments or decrements of addresses with memory accesses.
This pass is always skipped on architectures that do not have
instructions to support this. Enabled by default at -O and
higher on architectures that support this.

Attempt to transform conditional jumps into branch-less equivalents. This
include use of conditional moves, min, max, set flags and abs instructions, and
some tricks doable by standard arithmetics. The use of conditional execution
on chips where it is available is controlled by if-conversion2.

Assume that programs cannot safely dereference null pointers, and that
no code or data element resides there. This enables simple constant
folding optimizations at all optimization levels. In addition, other
optimization passes in GCC use this flag to control global dataflow
analyses that eliminate useless checks for null pointers; these assume
that if a pointer is checked after it has already been dereferenced,
it cannot be null.

Note however that in some environments this assumption is not true.
Use -fno-delete-null-pointer-checks to disable this optimization
for programs which depend on that behavior.

Some targets, especially embedded ones, disable this option at all levels.
Otherwise it is enabled at all levels: -O0, -O1,
-O2, -O3, -Os. Passes that use the information
are enabled independently at different optimization levels.

-fdevirtualize

Attempt to convert calls to virtual functions to direct calls. This
is done both within a procedure and interprocedurally as part of
indirect inlining (-findirect-inlining) and interprocedural constant
propagation (-fipa-cp).
Enabled at levels -O2, -O3, -Os.

-fexpensive-optimizations

Perform a number of minor optimizations that are relatively expensive.

Enabled at levels -O2, -O3, -Os.

-foptimize-register-move

-fregmove

Attempt to reassign register numbers in move instructions and as
operands of other simple instructions in order to maximize the amount of
register tying. This is especially helpful on machines with two-operand
instructions.

Note -fregmove and -foptimize-register-move are the same
optimization.

Enabled at levels -O2, -O3, -Os.

-fira-algorithm=algorithm

Use specified coloring algorithm for the integrated register
allocator. The algorithm argument should be priority or
CB. The first algorithm specifies Chow's priority coloring,
the second one specifies Chaitin-Briggs coloring. The second
algorithm can be unimplemented for some architectures. If it is
implemented, it is the default because Chaitin-Briggs coloring as a
rule generates a better code.

-fira-region=region

Use specified regions for the integrated register allocator. The
region argument should be one of all, mixed, or
one. The first value means using all loops as register
allocation regions, the second value which is the default means using
all loops except for loops with small register pressure as the
regions, and third one means using all function as a single region.
The first value can give best result for machines with small size and
irregular register set, the third one results in faster and generates
decent code and the smallest size code, and the default value usually
give the best results in most cases and for most architectures.

-fira-loop-pressure

Use IRA to evaluate register pressure in loops for decision to move
loop invariants. Usage of this option usually results in generation
of faster and smaller code on machines with big register files (>= 32
registers) but it can slow compiler down.

This option is enabled at level -O3 for some targets.

-fno-ira-share-save-slots

Switch off sharing stack slots used for saving call used hard
registers living through a call. Each hard register will get a
separate stack slot and as a result function stack frame will be
bigger.

-fno-ira-share-spill-slots

Switch off sharing stack slots allocated for pseudo-registers. Each
pseudo-register which did not get a hard register will get a separate
stack slot and as a result function stack frame will be bigger.

-fira-verbose=n

Set up how verbose dump file for the integrated register allocator
will be. Default value is 5. If the value is greater or equal to 10,
the dump file will be stderr as if the value were n minus 10.

-fdelayed-branch

If supported for the target machine, attempt to reorder instructions
to exploit instruction slots available after delayed branch
instructions.

Enabled at levels -O, -O2, -O3, -Os.

-fschedule-insns

If supported for the target machine, attempt to reorder instructions to
eliminate execution stalls due to required data being unavailable. This
helps machines that have slow floating point or memory load instructions
by allowing other instructions to be issued until the result of the load
or floating point instruction is required.

Enabled at levels -O2, -O3.

-fschedule-insns2

Similar to -fschedule-insns, but requests an additional pass of
instruction scheduling after register allocation has been done. This is
especially useful on machines with a relatively small number of
registers and where memory load instructions take more than one cycle.

Enabled at levels -O2, -O3, -Os.

-fno-sched-interblock

Don't schedule instructions across basic blocks. This is normally
enabled by default when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.

-fno-sched-spec

Don't allow speculative motion of non-load instructions. This is normally
enabled by default when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.

-fsched-pressure

Enable register pressure sensitive insn scheduling before the register
allocation. This only makes sense when scheduling before register
allocation is enabled, i.e. with -fschedule-insns or at
-O2 or higher. Usage of this option can improve the
generated code and decrease its size by preventing register pressure
increase above the number of available hard registers and as a
consequence register spills in the register allocation.

-fsched-spec-load

Allow speculative motion of some load instructions. This only makes
sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.

-fsched-spec-load-dangerous

Allow speculative motion of more load instructions. This only makes
sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.

-fsched-stalled-insns

-fsched-stalled-insns=n

Define how many insns (if any) can be moved prematurely from the queue
of stalled insns into the ready list, during the second scheduling pass.
-fno-sched-stalled-insns means that no insns will be moved
prematurely, -fsched-stalled-insns=0 means there is no limit
on how many queued insns can be moved prematurely.
-fsched-stalled-insns without a value is equivalent to
-fsched-stalled-insns=1.

-fsched-stalled-insns-dep

-fsched-stalled-insns-dep=n

Define how many insn groups (cycles) will be examined for a dependency
on a stalled insn that is candidate for premature removal from the queue
of stalled insns. This has an effect only during the second scheduling pass,
and only if -fsched-stalled-insns is used.
-fno-sched-stalled-insns-dep is equivalent to
-fsched-stalled-insns-dep=0.
-fsched-stalled-insns-dep without a value is equivalent to
-fsched-stalled-insns-dep=1.

-fsched2-use-superblocks

When scheduling after register allocation, do use superblock scheduling
algorithm. Superblock scheduling allows motion across basic block boundaries
resulting on faster schedules. This option is experimental, as not all machine
descriptions used by GCC model the CPU closely enough to avoid unreliable
results from the algorithm.

This only makes sense when scheduling after register allocation, i.e. with
-fschedule-insns2 or at -O2 or higher.

-fsched-group-heuristic

Enable the group heuristic in the scheduler. This heuristic favors
the instruction that belongs to a schedule group. This is enabled
by default when scheduling is enabled, i.e. with -fschedule-insns
or -fschedule-insns2 or at -O2 or higher.

-fsched-critical-path-heuristic

Enable the critical-path heuristic in the scheduler. This heuristic favors
instructions on the critical path. This is enabled by default when
scheduling is enabled, i.e. with -fschedule-insns
or -fschedule-insns2 or at -O2 or higher.

-fsched-spec-insn-heuristic

Enable the speculative instruction heuristic in the scheduler. This
heuristic favors speculative instructions with greater dependency weakness.
This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2
or at -O2 or higher.

-fsched-rank-heuristic

Enable the rank heuristic in the scheduler. This heuristic favors
the instruction belonging to a basic block with greater size or frequency.
This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2 or
at -O2 or higher.

-fsched-last-insn-heuristic

Enable the last-instruction heuristic in the scheduler. This heuristic
favors the instruction that is less dependent on the last instruction
scheduled. This is enabled by default when scheduling is enabled,
i.e. with -fschedule-insns or -fschedule-insns2 or
at -O2 or higher.

-fsched-dep-count-heuristic

Enable the dependent-count heuristic in the scheduler. This heuristic
favors the instruction that has more instructions depending on it.
This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2 or
at -O2 or higher.

-freschedule-modulo-scheduled-loops

The modulo scheduling comes before the traditional scheduling, if a loop
was modulo scheduled we may want to prevent the later scheduling passes
from changing its schedule, we use this option to control that.

Enable software pipelining of innermost loops during selective scheduling.
This option has no effect until one of -fselective-scheduling or
-fselective-scheduling2 is turned on.

-fsel-sched-pipelining-outer-loops

When pipelining loops during selective scheduling, also pipeline outer loops.
This option has no effect until -fsel-sched-pipelining is turned on.

-fcaller-saves

Enable values to be allocated in registers that will be clobbered by
function calls, by emitting extra instructions to save and restore the
registers around such calls. Such allocation is done only when it
seems to result in better code than would otherwise be produced.

This option is always enabled by default on certain machines, usually
those which have no call-preserved registers to use instead.

Enabled at levels -O2, -O3, -Os.

-fcombine-stack-adjustments

Tracks stack adjustments (pushes and pops) and stack memory references
and then tries to find ways to combine them.

Enabled by default at -O1 and higher.

-fconserve-stack

Attempt to minimize stack usage. The compiler will attempt to use less
stack space, even if that makes the program slower. This option
implies setting the large-stack-frame parameter to 100
and the large-stack-frame-growth parameter to 400.

-ftree-reassoc

Perform reassociation on trees. This flag is enabled by default
at -O and higher.

-ftree-pre

Perform partial redundancy elimination (PRE) on trees. This flag is
enabled by default at -O2 and -O3.

-ftree-forwprop

Perform forward propagation on trees. This flag is enabled by default
at -O and higher.

-ftree-fre

Perform full redundancy elimination (FRE) on trees. The difference
between FRE and PRE is that FRE only considers expressions
that are computed on all paths leading to the redundant computation.
This analysis is faster than PRE, though it exposes fewer redundancies.
This flag is enabled by default at -O and higher.

-ftree-phiprop

Perform hoisting of loads from conditional pointers on trees. This
pass is enabled by default at -O and higher.

-ftree-copy-prop

Perform copy propagation on trees. This pass eliminates unnecessary
copy operations. This flag is enabled by default at -O and
higher.

-fipa-pure-const

Discover which functions are pure or constant.
Enabled by default at -O and higher.

-fipa-reference

Discover which static variables do not escape cannot escape the
compilation unit.
Enabled by default at -O and higher.

-fipa-struct-reorg

Perform structure reorganization optimization, that change C-like structures
layout in order to better utilize spatial locality. This transformation is
affective for programs containing arrays of structures. Available in two
compilation modes: profile-based (enabled with -fprofile-generate)
or static (which uses built-in heuristics). It works only in whole program
mode, so it requires -fwhole-program to be
enabled. Structures considered `cold' by this transformation are not
affected (see --param struct-reorg-cold-struct-ratio=value).

With this flag, the program debug info reflects a new structure layout.

-fipa-pta

Perform interprocedural pointer analysis and interprocedural modification
and reference analysis. This option can cause excessive memory and
compile-time usage on large compilation units. It is not enabled by
default at any optimization level.

-fipa-profile

Perform interprocedural profile propagation. The functions called only from
cold functions are marked as cold. Also functions executed once (such as
cold, noreturn, static constructors or destructors) are identified. Cold
functions and loop less parts of functions executed once are then optimized for
size.
Enabled by default at -O and higher.

-fipa-cp

Perform interprocedural constant propagation.
This optimization analyzes the program to determine when values passed
to functions are constants and then optimizes accordingly.
This optimization can substantially increase performance
if the application has constants passed to functions.
This flag is enabled by default at -O2, -Os and -O3.

-fipa-cp-clone

Perform function cloning to make interprocedural constant propagation stronger.
When enabled, interprocedural constant propagation will perform function cloning
when externally visible function can be called with constant arguments.
Because this optimization can create multiple copies of functions,
it may significantly increase code size
(see --param ipcp-unit-growth=value).
This flag is enabled by default at -O3.

-fipa-matrix-reorg

Perform matrix flattening and transposing.
Matrix flattening tries to replace an m-dimensional matrix
with its equivalent n-dimensional matrix, where n < m.
This reduces the level of indirection needed for accessing the elements
of the matrix. The second optimization is matrix transposing that
attempts to change the order of the matrix's dimensions in order to
improve cache locality.
Both optimizations need the -fwhole-program flag.
Transposing is enabled only if profiling information is available.

-ftree-sink

Perform forward store motion on trees. This flag is
enabled by default at -O and higher.

-ftree-bit-ccp

Perform sparse conditional bit constant propagation on trees and propagate
pointer alignment information.
This pass only operates on local scalar variables and is enabled by default
at -O and higher. It requires that -ftree-ccp is enabled.

-ftree-ccp

Perform sparse conditional constant propagation (CCP) on trees. This
pass only operates on local scalar variables and is enabled by default
at -O and higher.

-ftree-switch-conversion

Perform conversion of simple initializations in a switch to
initializations from a scalar array. This flag is enabled by default
at -O2 and higher.

-ftree-dce

Perform dead code elimination (DCE) on trees. This flag is enabled by
default at -O and higher.

-ftree-builtin-call-dce

Perform conditional dead code elimination (DCE) for calls to builtin functions
that may set errno but are otherwise side-effect free. This flag is
enabled by default at -O2 and higher if -Os is not also
specified.

-ftree-dominator-opts

Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and expression
simplification) based on a dominator tree traversal. This also
performs jump threading (to reduce jumps to jumps). This flag is
enabled by default at -O and higher.

-ftree-dse

Perform dead store elimination (DSE) on trees. A dead store is a store into
a memory location which will later be overwritten by another store without
any intervening loads. In this case the earlier store can be deleted. This
flag is enabled by default at -O and higher.

-ftree-ch

Perform loop header copying on trees. This is beneficial since it increases
effectiveness of code motion optimizations. It also saves one jump. This flag
is enabled by default at -O and higher. It is not enabled
for -Os, since it usually increases code size.

-ftree-loop-optimize

Perform loop optimizations on trees. This flag is enabled by default
at -O and higher.

-ftree-loop-linear

Perform loop interchange transformations on tree. Same as
-floop-interchange. To use this code transformation, GCC has
to be configured with --with-ppl and --with-cloog to
enable the Graphite loop transformation infrastructure.

-floop-interchange

Perform loop interchange transformations on loops. Interchanging two
nested loops switches the inner and outer loops. For example, given a
loop like:

DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO

loop interchange will transform the loop as if the user had written:

DO I = 1, N
DO J = 1, M
A(J, I) = A(J, I) * C
ENDDO
ENDDO

which can be beneficial when N is larger than the caches,
because in Fortran, the elements of an array are stored in memory
contiguously by column, and the original loop iterates over rows,
potentially creating at each access a cache miss. This optimization
applies to all the languages supported by GCC and is not limited to
Fortran. To use this code transformation, GCC has to be configured
with --with-ppl and --with-cloog to enable the
Graphite loop transformation infrastructure.

-floop-strip-mine

Perform loop strip mining transformations on loops. Strip mining
splits a loop into two nested loops. The outer loop has strides
equal to the strip size and the inner loop has strides of the
original loop within a strip. The strip length can be changed
using the loop-block-tile-size parameter. For example,
given a loop like:

This optimization applies to all the languages supported by GCC and is
not limited to Fortran. To use this code transformation, GCC has to
be configured with --with-ppl and --with-cloog to
enable the Graphite loop transformation infrastructure.

-floop-block

Perform loop blocking transformations on loops. Blocking strip mines
each loop in the loop nest such that the memory accesses of the
element loops fit inside caches. The strip length can be changed
using the loop-block-tile-size parameter. For example, given
a loop like:

which can be beneficial when M is larger than the caches,
because the innermost loop will iterate over a smaller amount of data
that can be kept in the caches. This optimization applies to all the
languages supported by GCC and is not limited to Fortran. To use this
code transformation, GCC has to be configured with --with-ppl
and --with-cloog to enable the Graphite loop transformation
infrastructure.

-fgraphite-identity

Enable the identity transformation for graphite. For every SCoP we generate
the polyhedral representation and transform it back to gimple. Using
-fgraphite-identity we can check the costs or benefits of the
GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations
are also performed by the code generator CLooG, like index splitting and
dead code elimination in loops.

-floop-flatten

Removes the loop nesting structure: transforms the loop nest into a
single loop. This transformation can be useful to vectorize all the
levels of the loop nest.

-floop-parallelize-all

Use the Graphite data dependence analysis to identify loops that can
be parallelized. Parallelize all the loops that can be analyzed to
not contain loop carried dependences without checking that it is
profitable to parallelize the loops.

-fcheck-data-deps

Compare the results of several data dependence analyzers. This option
is used for debugging the data dependence analyzers.

-ftree-loop-if-convert

Attempt to transform conditional jumps in the innermost loops to
branch-less equivalents. The intent is to remove control-flow from
the innermost loops in order to improve the ability of the
vectorization pass to handle these loops. This is enabled by default
if vectorization is enabled.

-ftree-loop-if-convert-stores

Attempt to also if-convert conditional jumps containing memory writes.
This transformation can be unsafe for multi-threaded programs as it
transforms conditional memory writes into unconditional memory writes.
For example,

for (i = 0; i < N; i++)
if (cond)
A[i] = expr;

would be transformed to

for (i = 0; i < N; i++)
A[i] = cond ? expr : A[i];

potentially producing data races.

-ftree-loop-distribution

Perform loop distribution. This flag can improve cache performance on
big loop bodies and allow further loop optimizations, like
parallelization or vectorization, to take place. For example, the loop

DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO

is transformed to

DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO

-ftree-loop-distribute-patterns

Perform loop distribution of patterns that can be code generated with
calls to a library. This flag is enabled by default at -O3.

This pass distributes the initialization loops and generates a call to
memset zero. For example, the loop

DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO

is transformed to

DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO

and the initialization loop is transformed into a call to memset zero.

-ftree-loop-im

Perform loop invariant motion on trees. This pass moves only invariants that
would be hard to handle at RTL level (function calls, operations that expand to
nontrivial sequences of insns). With -funswitch-loops it also moves
operands of conditions that are invariant out of the loop, so that we can use
just trivial invariantness analysis in loop unswitching. The pass also includes
store motion.

-ftree-loop-ivcanon

Create a canonical counter for number of iterations in the loop for that
determining number of iterations requires complicated analysis. Later
optimizations then may determine the number easily. Useful especially
in connection with unrolling.

Parallelize loops, i.e., split their iteration space to run in n threads.
This is only possible for loops whose iterations are independent
and can be arbitrarily reordered. The optimization is only
profitable on multiprocessor machines, for loops that are CPU-intensive,
rather than constrained e.g. by memory bandwidth. This option
implies -pthread, and thus is only supported on targets
that have support for -pthread.

-ftree-pta

Perform function-local points-to analysis on trees. This flag is
enabled by default at -O and higher.

-ftree-sra

Perform scalar replacement of aggregates. This pass replaces structure
references with scalars to prevent committing structures to memory too
early. This flag is enabled by default at -O and higher.

-ftree-copyrename

Perform copy renaming on trees. This pass attempts to rename compiler
temporaries to other variables at copy locations, usually resulting in
variable names which more closely resemble the original variables. This flag
is enabled by default at -O and higher.

-ftree-ter

Perform temporary expression replacement during the SSA->normal phase. Single
use/single def temporaries are replaced at their use location with their
defining expression. This results in non-GIMPLE code, but gives the expanders
much more complex trees to work on resulting in better RTL generation. This is
enabled by default at -O and higher.

-ftree-vectorize

Perform loop vectorization on trees. This flag is enabled by default at
-O3.

-ftree-slp-vectorize

Perform basic block vectorization on trees. This flag is enabled by default at
-O3 and when -ftree-vectorize is enabled.

-ftree-vect-loop-version

Perform loop versioning when doing loop vectorization on trees. When a loop
appears to be vectorizable except that data alignment or data dependence cannot
be determined at compile time then vectorized and non-vectorized versions of
the loop are generated along with runtime checks for alignment or dependence
to control which version is executed. This option is enabled by default
except at level -Os where it is disabled.

-fvect-cost-model

Enable cost model for vectorization.

-ftree-vrp

Perform Value Range Propagation on trees. This is similar to the
constant propagation pass, but instead of values, ranges of values are
propagated. This allows the optimizers to remove unnecessary range
checks like array bound checks and null pointer checks. This is
enabled by default at -O2 and higher. Null pointer check
elimination is only done if -fdelete-null-pointer-checks is
enabled.

-ftracer

Perform tail duplication to enlarge superblock size. This transformation
simplifies the control flow of the function allowing other optimizations to do
better job.

-funroll-loops

Unroll loops whose number of iterations can be determined at compile
time or upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop. This option makes code larger,
and may or may not make it run faster.

-funroll-all-loops

Unroll all loops, even if their number of iterations is uncertain when
the loop is entered. This usually makes programs run more slowly.
-funroll-all-loops implies the same options as
-funroll-loops,

-fsplit-ivs-in-unroller

Enables expressing of values of induction variables in later iterations
of the unrolled loop using the value in the first iteration. This breaks
long dependency chains, thus improving efficiency of the scheduling passes.

Combination of -fweb and CSE is often sufficient to obtain the
same effect. However in cases the loop body is more complicated than
a single basic block, this is not reliable. It also does not work at all
on some of the architectures due to restrictions in the CSE pass.

This optimization is enabled by default.

-fvariable-expansion-in-unroller

With this option, the compiler will create multiple copies of some
local variables when unrolling a loop which can result in superior code.

-fpartial-inlining

Inline parts of functions. This option has any effect only
when inlining itself is turned on by the -finline-functions
or -finline-small-functions options.

If supported by the target machine, generate instructions to prefetch
memory to improve the performance of loops that access large arrays.

This option may generate better or worse code; results are highly
dependent on the structure of loops within the source code.

Disabled at level -Os.

-fno-peephole

-fno-peephole2

Disable any machine-specific peephole optimizations. The difference
between -fno-peephole and -fno-peephole2 is in how they
are implemented in the compiler; some targets use one, some use the
other, a few use both.

GCC will use heuristics to guess branch probabilities if they are
not provided by profiling feedback (-fprofile-arcs). These
heuristics are based on the control flow graph. If some branch probabilities
are specified by `__builtin_expect', then the heuristics will be
used to guess branch probabilities for the rest of the control flow graph,
taking the `__builtin_expect' info into account. The interactions
between the heuristics and `__builtin_expect' can be complex, and in
some cases, it may be useful to disable the heuristics so that the effects
of `__builtin_expect' are easier to understand.

The default is -fguess-branch-probability at levels
-O, -O2, -O3, -Os.

-freorder-blocks

Reorder basic blocks in the compiled function in order to reduce number of
taken branches and improve code locality.

Enabled at levels -O2, -O3.

-freorder-blocks-and-partition

In addition to reordering basic blocks in the compiled function, in order
to reduce number of taken branches, partitions hot and cold basic blocks
into separate sections of the assembly and .o files, to improve
paging and cache locality performance.

This optimization is automatically turned off in the presence of
exception handling, for linkonce sections, for functions with a user-defined
section attribute and on any architecture that does not support named
sections.

-freorder-functions

Reorder functions in the object file in order to
improve code locality. This is implemented by using special
subsections .text.hot for most frequently executed functions and
.text.unlikely for unlikely executed functions. Reordering is done by
the linker so object file format must support named sections and linker must
place them in a reasonable way.

Also profile feedback must be available in to make this option effective. See
-fprofile-arcs for details.

Enabled at levels -O2, -O3, -Os.

-fstrict-aliasing

Allow the compiler to assume the strictest aliasing rules applicable to
the language being compiled. For C (and C++), this activates
optimizations based on the type of expressions. In particular, an
object of one type is assumed never to reside at the same address as an
object of a different type, unless the types are almost the same. For
example, an unsigned int can alias an int, but not a
void* or a double. A character type may alias any other
type.

The practice of reading from a different union member than the one most
recently written to (called “type-punning”) is common. Even with
-fstrict-aliasing, type-punning is allowed, provided the memory
is accessed through the union type. So, the code above will work as
expected. See Structures unions enumerations and bit-fields implementation. However, this code might not:

Similarly, access by taking the address, casting the resulting pointer
and dereferencing the result has undefined behavior, even if the cast
uses a union type, e.g.:

int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}

The -fstrict-aliasing option is enabled at levels
-O2, -O3, -Os.

-fstrict-overflow

Allow the compiler to assume strict signed overflow rules, depending
on the language being compiled. For C (and C++) this means that
overflow when doing arithmetic with signed numbers is undefined, which
means that the compiler may assume that it will not happen. This
permits various optimizations. For example, the compiler will assume
that an expression like i + 10 > i will always be true for
signed i. This assumption is only valid if signed overflow is
undefined, as the expression is false if i + 10 overflows when
using twos complement arithmetic. When this option is in effect any
attempt to determine whether an operation on signed numbers will
overflow must be written carefully to not actually involve overflow.

This option also allows the compiler to assume strict pointer
semantics: given a pointer to an object, if adding an offset to that
pointer does not produce a pointer to the same object, the addition is
undefined. This permits the compiler to conclude that p + u >
p is always true for a pointer p and unsigned integer
u. This assumption is only valid because pointer wraparound is
undefined, as the expression is false if p + u overflows using
twos complement arithmetic.

See also the -fwrapv option. Using -fwrapv means
that integer signed overflow is fully defined: it wraps. When
-fwrapv is used, there is no difference between
-fstrict-overflow and -fno-strict-overflow for
integers. With -fwrapv certain types of overflow are
permitted. For example, if the compiler gets an overflow when doing
arithmetic on constants, the overflowed value can still be used with
-fwrapv, but not otherwise.

The -fstrict-overflow option is enabled at levels
-O2, -O3, -Os.

-falign-functions

-falign-functions=n

Align the start of functions to the next power-of-two greater than
n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to the next 32-byte
boundary, but -falign-functions=24 would align to the next
32-byte boundary only if this can be done by skipping 23 bytes or less.

-fno-align-functions and -falign-functions=1 are
equivalent and mean that functions will not be aligned.

Some assemblers only support this flag when n is a power of two;
in that case, it is rounded up.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-falign-labels

-falign-labels=n

Align all branch targets to a power-of-two boundary, skipping up to
n bytes like -falign-functions. This option can easily
make code slower, because it must insert dummy operations for when the
branch target is reached in the usual flow of the code.

-fno-align-labels and -falign-labels=1 are
equivalent and mean that labels will not be aligned.

If -falign-loops or -falign-jumps are applicable and
are greater than this value, then their values are used instead.

If n is not specified or is zero, use a machine-dependent default
which is very likely to be `1', meaning no alignment.

Enabled at levels -O2, -O3.

-falign-loops

-falign-loops=n

Align loops to a power-of-two boundary, skipping up to n bytes
like -falign-functions. The hope is that the loop will be
executed many times, which will make up for any execution of the dummy
operations.

-fno-align-loops and -falign-loops=1 are
equivalent and mean that loops will not be aligned.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-falign-jumps

-falign-jumps=n

Align branch targets to a power-of-two boundary, for branch targets
where the targets can only be reached by jumping, skipping up to n
bytes like -falign-functions. In this case, no dummy operations
need be executed.

-fno-align-jumps and -falign-jumps=1 are
equivalent and mean that loops will not be aligned.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

-funit-at-a-time

This option is left for compatibility reasons. -funit-at-a-time
has no effect, while -fno-unit-at-a-time implies
-fno-toplevel-reorder and -fno-section-anchors.

Enabled by default.

-fno-toplevel-reorder

Do not reorder top-level functions, variables, and asm
statements. Output them in the same order that they appear in the
input file. When this option is used, unreferenced static variables
will not be removed. This option is intended to support existing code
which relies on a particular ordering. For new code, it is better to
use attributes.

Enabled at level -O0. When disabled explicitly, it also imply
-fno-section-anchors that is otherwise enabled at -O0 on some
targets.

-fweb

Constructs webs as commonly used for register allocation purposes and assign
each web individual pseudo register. This allows the register allocation pass
to operate on pseudos directly, but also strengthens several other optimization
passes, such as CSE, loop optimizer and trivial dead code remover. It can,
however, make debugging impossible, since variables will no longer stay in a
“home register”.

Enabled by default with -funroll-loops.

-fwhole-program

Assume that the current compilation unit represents the whole program being
compiled. All public functions and variables with the exception of main
and those merged by attribute externally_visible become static functions
and in effect are optimized more aggressively by interprocedural optimizers. If gold is used as the linker plugin, externally_visible attributes are automatically added to functions (not variable yet due to a current gold issue) that are accessed outside of LTO objects according to resolution file produced by gold. For other linkers that cannot generate resolution file, explicit externally_visible attributes are still necessary.
While this option is equivalent to proper use of the static keyword for
programs consisting of a single file, in combination with option
-flto this flag can be used to
compile many smaller scale programs since the functions and variables become
local for the whole combined compilation unit, not for the single source file
itself.

This option implies -fwhole-file for Fortran programs.

-flto[=n]

This option runs the standard link-time optimizer. When invoked
with source code, it generates GIMPLE (one of GCC's internal
representations) and writes it to special ELF sections in the object
file. When the object files are linked together, all the function
bodies are read from these ELF sections and instantiated as if they
had been part of the same translation unit.

To use the link-timer optimizer, -flto needs to be specified at
compile time and during the final link. For example,

The first two invocations to GCC will save a bytecode representation
of GIMPLE into special ELF sections inside foo.o and
bar.o. The final invocation will read the GIMPLE bytecode from
foo.o and bar.o, merge the two files into a single
internal image, and compile the result as usual. Since both
foo.o and bar.o are merged into a single image, this
causes all the inter-procedural analyses and optimizations in GCC to
work across the two files as if they were a single one. This means,
for example, that the inliner will be able to inline functions in
bar.o into functions in foo.o and vice-versa.

Another (simpler) way to enable link-time optimization is,

gcc -o myprog -flto -O2 foo.c bar.c

The above will generate bytecode for foo.c and bar.c,
merge them together into a single GIMPLE representation and optimize
them as usual to produce myprog.

The only important thing to keep in mind is that to enable link-time
optimizations the -flto flag needs to be passed to both the
compile and the link commands.

To make whole program optimization effective, it is necessary to make
certain whole program assumptions. The compiler needs to know
what functions and variables can be accessed by libraries and runtime
outside of the link time optimized unit. When supported by the linker,
the linker plugin (see -fuse-linker-plugin) passes to the
compiler information about used and externally visible symbols. When
the linker plugin is not available, -fwhole-program should be
used to allow the compiler to make these assumptions, which will lead
to more aggressive optimization decisions.

Note that when a file is compiled with -flto, the generated
object file will be larger than a regular object file because it will
contain GIMPLE bytecodes and the usual final code. This means that
object files with LTO information can be linked as a normal object
file. So, in the previous example, if the final link is done with

gcc -o myprog foo.o bar.o

The only difference will be that no inter-procedural optimizations
will be applied to produce myprog. The two object files
foo.o and bar.o will be simply sent to the regular
linker.

Additionally, the optimization flags used to compile individual files
are not necessarily related to those used at link-time. For instance,

This will produce individual object files with unoptimized assembler
code, but the resulting binary myprog will be optimized at
-O3. Now, if the final binary is generated without
-flto, then myprog will not be optimized.

When producing the final binary with -flto, GCC will only
apply link-time optimizations to those files that contain bytecode.
Therefore, you can mix and match object files and libraries with
GIMPLE bytecodes and final object code. GCC will automatically select
which files to optimize in LTO mode and which files to link without
further processing.

There are some code generation flags that GCC will preserve when
generating bytecodes, as they need to be used during the final link
stage. Currently, the following options are saved into the GIMPLE
bytecode files: -fPIC, -fcommon and all the
-m target flags.

At link time, these options are read-in and reapplied. Note that the
current implementation makes no attempt at recognizing conflicting
values for these options. If two or more files have a conflicting
value (e.g., one file is compiled with -fPIC and another
isn't), the compiler will simply use the last value read from the
bytecode files. It is recommended, then, that all the files
participating in the same link be compiled with the same options.

Another feature of LTO is that it is possible to apply interprocedural
optimizations on files written in different languages. This requires
some support in the language front end. Currently, the C, C++ and
Fortran front ends are capable of emitting GIMPLE bytecodes, so
something like this should work

Notice that the final link is done with g++ to get the C++
runtime libraries and -lgfortran is added to get the Fortran
runtime libraries. In general, when mixing languages in LTO mode, you
should use the same link command used when mixing languages in a
regular (non-LTO) compilation. This means that if your build process
was mixing languages before, all you need to add is -flto to
all the compile and link commands.

If LTO encounters objects with C linkage declared with incompatible
types in separate translation units to be linked together (undefined
behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be
issued. The behavior is still undefined at runtime.

If object files containing GIMPLE bytecode are stored in a library archive, say
libfoo.a, it is possible to extract and use them in an LTO link if you
are using a linker with linker plugin support. To enable this feature, use
the flag -fuse-linker-plugin at link-time:

gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo

With the linker plugin enabled, the linker will extract the needed
GIMPLE files from libfoo.a and pass them on to the running GCC
to make them part of the aggregated GIMPLE image to be optimized.

If you are not using a linker with linker plugin support and/or do not
enable linker plugin then the objects inside libfoo.a
will be extracted and linked as usual, but they will not participate
in the LTO optimization process.

Link time optimizations do not require the presence of the whole program to
operate. If the program does not require any symbols to be exported, it is
possible to combine -flto and with -fwhole-program to allow
the interprocedural optimizers to use more aggressive assumptions which may
lead to improved optimization opportunities.
Use of -fwhole-program is not needed when linker plugin is
active (see -fuse-linker-plugin).

Regarding portability: the current implementation of LTO makes no
attempt at generating bytecode that can be ported between different
types of hosts. The bytecode files are versioned and there is a
strict version check, so bytecode files generated in one version of
GCC will not work with an older/newer version of GCC.

Link time optimization does not play well with generating debugging
information. Combining -flto with
-g is currently experimental and expected to produce wrong
results.

If you specify the optional n, the optimization and code
generation done at link time is executed in parallel using n
parallel jobs by utilizing an installed make program. The
environment variable MAKE may be used to override the program
used. The default value for n is 1.

You can also specify -flto=jobserver to use GNU make's
job server mode to determine the number of parallel jobs. This
is useful when the Makefile calling GCC is already executing in parallel.
The parent Makefile will need a `+' prepended to the command recipe
for this to work. This will likely only work if MAKE is
GNU make.

This option is disabled by default.

-flto-partition=alg

Specify the partitioning algorithm used by the link time optimizer.
The value is either 1to1 to specify a partitioning mirroring
the original source files or balanced to specify partitioning
into equally sized chunks (whenever possible). Specifying none
as an algorithm disables partitioning and streaming completely. The
default value is balanced.

-flto-compression-level=n

This option specifies the level of compression used for intermediate
language written to LTO object files, and is only meaningful in
conjunction with LTO mode (-flto). Valid
values are 0 (no compression) to 9 (maximum compression). Values
outside this range are clamped to either 0 or 9. If the option is not
given, a default balanced compression setting is used.

-flto-report

Prints a report with internal details on the workings of the link-time
optimizer. The contents of this report vary from version to version,
it is meant to be useful to GCC developers when processing object
files in LTO mode (via -flto).

Disabled by default.

-fuse-linker-plugin

Enables the use of linker plugin during link time optimization. This option
relies on the linker plugin support in linker that is available in gold
or in GNU ld 2.21 or newer.

This option enables the extraction of object files with GIMPLE bytecode out of
library archives. This improves the quality of optimization by exposing more
code the the link time optimizer. This information specify what symbols
can be accessed externally (by non-LTO object or during dynamic linking).
Resulting code quality improvements on binaries (and shared libraries that do
use hidden visibility) is similar to -fwhole-program. See
-flto for a description on the effect of this flag and how to use it.

Enabled by default when LTO support in GCC is enabled and GCC was compiled
with a linker supporting plugins (GNU ld 2.21 or newer or gold).

-fcompare-elim

After register allocation and post-register allocation instruction splitting,
identify arithmetic instructions that compute processor flags similar to a
comparison operation based on that arithmetic. If possible, eliminate the
explicit comparison operation.

This pass only applies to certain targets that cannot explicitly represent
the comparison operation before register allocation is complete.

Enabled at levels -O, -O2, -O3, -Os.

-fcprop-registers

After register allocation and post-register allocation instruction splitting,
we perform a copy-propagation pass to try to reduce scheduling dependencies
and occasionally eliminate the copy.

Enabled at levels -O, -O2, -O3, -Os.

-fprofile-correction

Profiles collected using an instrumented binary for multi-threaded programs may
be inconsistent due to missed counter updates. When this option is specified,
GCC will use heuristics to correct or smooth out such inconsistencies. By
default, GCC will emit an error message when an inconsistent profile is detected.

-fprofile-dir=path

Set the directory to search for the profile data files in to path.
This option affects only the profile data generated by
-fprofile-generate, -ftest-coverage, -fprofile-arcs
and used by -fprofile-use and -fbranch-probabilities
and its related options.
By default, GCC will use the current directory as path, thus the
profile data file will appear in the same directory as the object file.

-fprofile-generate

-fprofile-generate=path

Enable options usually used for instrumenting application to produce
profile useful for later recompilation with profile feedback based
optimization. You must use -fprofile-generate both when
compiling and when linking your program.

The following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.

If path is specified, GCC will look at the path to find
the profile feedback data files. See -fprofile-dir.

The following options are enabled: -fbranch-probabilities, -fvpt,
-funroll-loops, -fpeel-loops, -ftracer

By default, GCC emits an error message if the feedback profiles do not
match the source code. This error can be turned into a warning by using
-Wcoverage-mismatch. Note this may result in poorly optimized
code.

If path is specified, GCC will look at the path to find
the profile feedback data files. See -fprofile-dir.

The following options control compiler behavior regarding floating
point arithmetic. These options trade off between speed and
correctness. All must be specifically enabled.

-ffloat-store

Do not store floating point variables in registers, and inhibit other
options that might change whether a floating point value is taken from a
register or memory.

This option prevents undesirable excess precision on machines such as
the 68000 where the floating registers (of the 68881) keep more
precision than a double is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does only
good, but a few programs rely on the precise definition of IEEE floating
point. Use -ffloat-store for such programs, after modifying
them to store all pertinent intermediate computations into variables.

-fexcess-precision=style

This option allows further control over excess precision on machines
where floating-point registers have more precision than the IEEE
float and double types and the processor does not
support operations rounding to those types. By default,
-fexcess-precision=fast is in effect; this means that
operations are carried out in the precision of the registers and that
it is unpredictable when rounding to the types specified in the source
code takes place. When compiling C, if
-fexcess-precision=standard is specified then excess
precision will follow the rules specified in ISO C99; in particular,
both casts and assignments cause values to be rounded to their
semantic types (whereas -ffloat-store only affects
assignments). This option is enabled by default for C if a strict
conformance option such as -std=c99 is used.

-fexcess-precision=standard is not implemented for languages
other than C, and has no effect if
-funsafe-math-optimizations or -ffast-math is
specified. On the x86, it also has no effect if -mfpmath=sse
or -mfpmath=sse+387 is specified; in the former case, IEEE
semantics apply without excess precision, and in the latter, rounding
is unpredictable.

This option causes the preprocessor macro __FAST_MATH__ to be defined.

This option is not turned on by any -O option besides
-Ofast since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO rules/specifications
for math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.

-fno-math-errno

Do not set ERRNO after calling math functions that are executed
with a single instruction, e.g., sqrt. A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.

This option is not turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.

The default is -fmath-errno.

On Darwin systems, the math library never sets errno. There is
therefore no reason for the compiler to consider the possibility that
it might, and -fno-math-errno is the default.

-funsafe-math-optimizations

Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link-time, it may include libraries
or startup files that change the default FPU control word or other
similar optimizations.

This option is not turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.
Enables -fno-signed-zeros, -fno-trapping-math,
-fassociative-math and -freciprocal-math.

The default is -fno-unsafe-math-optimizations.

-fassociative-math

Allow re-association of operands in series of floating-point operations.
This violates the ISO C and C++ language standard by possibly changing
computation result. NOTE: re-ordering may change the sign of zero as
well as ignore NaNs and inhibit or create underflow or overflow (and
thus cannot be used on a code which relies on rounding behavior like
(x + 2**52) - 2**52). May also reorder floating-point comparisons
and thus may not be used when ordered comparisons are required.
This option requires that both -fno-signed-zeros and
-fno-trapping-math be in effect. Moreover, it doesn't make
much sense with -frounding-math. For Fortran the option
is automatically enabled when both -fno-signed-zeros and
-fno-trapping-math are in effect.

The default is -fno-associative-math.

-freciprocal-math

Allow the reciprocal of a value to be used instead of dividing by
the value if this enables optimizations. For example x / y
can be replaced with x * (1/y) which is useful if (1/y)
is subject to common subexpression elimination. Note that this loses
precision and increases the number of flops operating on the value.

The default is -fno-reciprocal-math.

-ffinite-math-only

Allow optimizations for floating-point arithmetic that assume
that arguments and results are not NaNs or +-Infs.

This option is not turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.

The default is -fno-finite-math-only.

-fno-signed-zeros

Allow optimizations for floating point arithmetic that ignore the
signedness of zero. IEEE arithmetic specifies the behavior of
distinct +0.0 and −0.0 values, which then prohibits simplification
of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only).
This option implies that the sign of a zero result isn't significant.

The default is -fsigned-zeros.

-fno-trapping-math

Compile code assuming that floating-point operations cannot generate
user-visible traps. These traps include division by zero, overflow,
underflow, inexact result and invalid operation. This option requires
that -fno-signaling-nans be in effect. Setting this option may
allow faster code if one relies on “non-stop” IEEE arithmetic, for example.

This option should never be turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions.

The default is -ftrapping-math.

-frounding-math

Disable transformations and optimizations that assume default floating
point rounding behavior. This is round-to-zero for all floating point
to integer conversions, and round-to-nearest for all other arithmetic
truncations. This option should be specified for programs that change
the FP rounding mode dynamically, or that may be executed with a
non-default rounding mode. This option disables constant folding of
floating point expressions at compile-time (which may be affected by
rounding mode) and arithmetic transformations that are unsafe in the
presence of sign-dependent rounding modes.

The default is -fno-rounding-math.

This option is experimental and does not currently guarantee to
disable all GCC optimizations that are affected by rounding mode.
Future versions of GCC may provide finer control of this setting
using C99's FENV_ACCESS pragma. This command line option
will be used to specify the default state for FENV_ACCESS.

-fsignaling-nans

Compile code assuming that IEEE signaling NaNs may generate user-visible
traps during floating-point operations. Setting this option disables
optimizations that may change the number of exceptions visible with
signaling NaNs. This option implies -ftrapping-math.

This option causes the preprocessor macro __SUPPORT_SNAN__ to
be defined.

The default is -fno-signaling-nans.

This option is experimental and does not currently guarantee to
disable all GCC optimizations that affect signaling NaN behavior.

-fsingle-precision-constant

Treat floating point constant as single precision constant instead of
implicitly converting it to double precision constant.

-fcx-limited-range

When enabled, this option states that a range reduction step is not
needed when performing complex division. Also, there is no checking
whether the result of a complex multiplication or division is NaN
+ I*NaN, with an attempt to rescue the situation in that case. The
default is -fno-cx-limited-range, but is enabled by
-ffast-math.

This option controls the default setting of the ISO C99
CX_LIMITED_RANGE pragma. Nevertheless, the option applies to
all languages.

-fcx-fortran-rules

Complex multiplication and division follow Fortran rules. Range
reduction is done as part of complex division, but there is no checking
whether the result of a complex multiplication or division is NaN
+ I*NaN, with an attempt to rescue the situation in that case.

The default is -fno-cx-fortran-rules.

The following options control optimizations that may improve
performance, but are not enabled by any -O options. This
section includes experimental options that may produce broken code.

-fbranch-probabilities

After running a program compiled with -fprofile-arcs
(see Options for Debugging Your Program or gcc), you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on
the number of times each branch was taken. When the program
compiled with -fprofile-arcs exits it saves arc execution
counts to a file called sourcename.gcda for each source
file. The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code
and the same optimization options for both compilations.

With -fbranch-probabilities, GCC puts a
`REG_BR_PROB' note on each `JUMP_INSN' and `CALL_INSN'.
These can be used to improve optimization. Currently, they are only
used in one place: in reorg.c, instead of guessing which path a
branch is most likely to take, the `REG_BR_PROB' values are used to
exactly determine which path is taken more often.

-fprofile-values

If combined with -fprofile-arcs, it adds code so that some
data about values of expressions in the program is gathered.

With -fbranch-probabilities, it reads back the data gathered
from profiling values of expressions for usage in optimizations.

Enabled with -fprofile-generate and -fprofile-use.

-fvpt

If combined with -fprofile-arcs, it instructs the compiler to add
a code to gather information about values of expressions.

With -fbranch-probabilities, it reads back the data gathered
and actually performs the optimizations based on them.
Currently the optimizations include specialization of division operation
using the knowledge about the value of the denominator.

-frename-registers

Attempt to avoid false dependencies in scheduled code by making use
of registers left over after register allocation. This optimization
will most benefit processors with lots of registers. Depending on the
debug information format adopted by the target, however, it can
make debugging impossible, since variables will no longer stay in
a “home register”.

Enabled by default with -funroll-loops and -fpeel-loops.

-ftracer

Perform tail duplication to enlarge superblock size. This transformation
simplifies the control flow of the function allowing other optimizations to do
better job.

Enabled with -fprofile-use.

-funroll-loops

Unroll loops whose number of iterations can be determined at compile time or
upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop, -fweb and -frename-registers.
It also turns on complete loop peeling (i.e. complete removal of loops with
small constant number of iterations). This option makes code larger, and may
or may not make it run faster.

Enabled with -fprofile-use.

-funroll-all-loops

Unroll all loops, even if their number of iterations is uncertain when
the loop is entered. This usually makes programs run more slowly.
-funroll-all-loops implies the same options as
-funroll-loops.

-fpeel-loops

Peels the loops for that there is enough information that they do not
roll much (from profile feedback). It also turns on complete loop peeling
(i.e. complete removal of loops with small constant number of iterations).

Move branches with loop invariant conditions out of the loop, with duplicates
of the loop on both branches (modified according to result of the condition).

-ffunction-sections

-fdata-sections

Place each function or data item into its own section in the output
file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name
in the output file.

Use these options on systems where the linker can perform optimizations
to improve locality of reference in the instruction space. Most systems
using the ELF object format and SPARC processors running Solaris 2 have
linkers with such optimizations. AIX may have these optimizations in
the future.

Only use these options when there are significant benefits from doing
so. When you specify these options, the assembler and linker will
create larger object and executable files and will also be slower.
You will not be able to use gprof on all systems if you
specify this option and you may have problems with debugging if
you specify both this option and -g.

-fbranch-target-load-optimize

Perform branch target register load optimization before prologue / epilogue
threading.
The use of target registers can typically be exposed only during reload,
thus hoisting loads out of loops and doing inter-block scheduling needs
a separate optimization pass.

Emit extra code to check for buffer overflows, such as stack smashing
attacks. This is done by adding a guard variable to functions with
vulnerable objects. This includes functions that call alloca, and
functions with buffers larger than 8 bytes. The guards are initialized
when a function is entered and then checked when the function exits.
If a guard check fails, an error message is printed and the program exits.

-fstack-protector-all

Like -fstack-protector except that all functions are protected.

-fsection-anchors

Try to reduce the number of symbolic address calculations by using
shared “anchor” symbols to address nearby objects. This transformation
can help to reduce the number of GOT entries and GOT accesses on some
targets.

For example, the implementation of the following function foo:

static int a, b, c;
int foo (void) { return a + b + c; }

would usually calculate the addresses of all three variables, but if you
compile it with -fsection-anchors, it will access the variables
from a common anchor point instead. The effect is similar to the
following pseudocode (which isn't valid C):

In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC will not inline functions
that contain more that a certain number of instructions. You can
control some of these constants on the command-line using the
--param option.

The names of specific parameters, and the meaning of the values, are
tied to the internals of the compiler, and are subject to change
without notice in future releases.

In each case, the value is an integer. The allowable choices for
name are given in the following table:

struct-reorg-cold-struct-ratio

The threshold ratio (as a percentage) between a structure frequency
and the frequency of the hottest structure in the program. This parameter
is used by struct-reorg optimization enabled by -fipa-struct-reorg.
We say that if the ratio of a structure frequency, calculated by profiling,
to the hottest structure frequency in the program is less than this
parameter, then structure reorganization is not applied to this structure.
The default is 10.

predictable-branch-outcome

When branch is predicted to be taken with probability lower than this threshold
(in percent), then it is considered well predictable. The default is 10.

max-crossjump-edges

The maximum number of incoming edges to consider for crossjumping.
The algorithm used by -fcrossjumping is O(N^2) in
the number of edges incoming to each block. Increasing values mean
more aggressive optimization, making the compile time increase with
probably small improvement in executable size.

min-crossjump-insns

The minimum number of instructions which must be matched at the end
of two blocks before crossjumping will be performed on them. This
value is ignored in the case where all instructions in the block being
crossjumped from are matched. The default value is 5.

max-grow-copy-bb-insns

The maximum code size expansion factor when copying basic blocks
instead of jumping. The expansion is relative to a jump instruction.
The default value is 8.

max-goto-duplication-insns

The maximum number of instructions to duplicate to a block that jumps
to a computed goto. To avoid O(N^2) behavior in a number of
passes, GCC factors computed gotos early in the compilation process,
and unfactors them as late as possible. Only computed jumps at the
end of a basic blocks with no more than max-goto-duplication-insns are
unfactored. The default value is 8.

max-delay-slot-insn-search

The maximum number of instructions to consider when looking for an
instruction to fill a delay slot. If more than this arbitrary number of
instructions is searched, the time savings from filling the delay slot
will be minimal so stop searching. Increasing values mean more
aggressive optimization, making the compile time increase with probably
small improvement in executable run time.

max-delay-slot-live-search

When trying to fill delay slots, the maximum number of instructions to
consider when searching for a block with valid live register
information. Increasing this arbitrarily chosen value means more
aggressive optimization, increasing the compile time. This parameter
should be removed when the delay slot code is rewritten to maintain the
control-flow graph.

max-gcse-memory

The approximate maximum amount of memory that will be allocated in
order to perform the global common subexpression elimination
optimization. If more memory than specified is required, the
optimization will not be done.

max-gcse-insertion-ratio

If the ratio of expression insertions to deletions is larger than this value
for any expression, then RTL PRE will insert or remove the expression and thus
leave partially redundant computations in the instruction stream. The default value is 20.

max-pending-list-length

The maximum number of pending dependencies scheduling will allow
before flushing the current state and starting over. Large functions
with few branches or calls can create excessively large lists which
needlessly consume memory and resources.

max-inline-insns-single

Several parameters control the tree inliner used in gcc.
This number sets the maximum number of instructions (counted in GCC's
internal representation) in a single function that the tree inliner
will consider for inlining. This only affects functions declared
inline and methods implemented in a class declaration (C++).
The default value is 400.

max-inline-insns-auto

When you use -finline-functions (included in -O3),
a lot of functions that would otherwise not be considered for inlining
by the compiler will be investigated. To those functions, a different
(more restrictive) limit compared to functions declared inline can
be applied.
The default value is 40.

large-function-insns

The limit specifying really large functions. For functions larger than this
limit after inlining, inlining is constrained by
--param large-function-growth. This parameter is useful primarily
to avoid extreme compilation time caused by non-linear algorithms used by the
backend.
The default value is 2700.

large-function-growth

Specifies maximal growth of large function caused by inlining in percents.
The default value is 100 which limits large function growth to 2.0 times
the original size.

large-unit-insns

The limit specifying large translation unit. Growth caused by inlining of
units larger than this limit is limited by --param inline-unit-growth.
For small units this might be too tight (consider unit consisting of function A
that is inline and B that just calls A three time. If B is small relative to
A, the growth of unit is 300\% and yet such inlining is very sane. For very
large units consisting of small inlineable functions however the overall unit
growth limit is needed to avoid exponential explosion of code size. Thus for
smaller units, the size is increased to --param large-unit-insns
before applying --param inline-unit-growth. The default is 10000

inline-unit-growth

Specifies maximal overall growth of the compilation unit caused by inlining.
The default value is 30 which limits unit growth to 1.3 times the original
size.

ipcp-unit-growth

Specifies maximal overall growth of the compilation unit caused by
interprocedural constant propagation. The default value is 10 which limits
unit growth to 1.1 times the original size.

large-stack-frame

The limit specifying large stack frames. While inlining the algorithm is trying
to not grow past this limit too much. Default value is 256 bytes.

large-stack-frame-growth

Specifies maximal growth of large stack frames caused by inlining in percents.
The default value is 1000 which limits large stack frame growth to 11 times
the original size.

max-inline-insns-recursive

max-inline-insns-recursive-auto

Specifies maximum number of instructions out-of-line copy of self recursive inline
function can grow into by performing recursive inlining.

For functions declared inline --param max-inline-insns-recursive is
taken into account. For function not declared inline, recursive inlining
happens only when -finline-functions (included in -O3) is
enabled and --param max-inline-insns-recursive-auto is used. The
default value is 450.

max-inline-recursive-depth

max-inline-recursive-depth-auto

Specifies maximum recursion depth used by the recursive inlining.

For functions declared inline --param max-inline-recursive-depth is
taken into account. For function not declared inline, recursive inlining
happens only when -finline-functions (included in -O3) is
enabled and --param max-inline-recursive-depth-auto is used. The
default value is 8.

min-inline-recursive-probability

Recursive inlining is profitable only for function having deep recursion
in average and can hurt for function having little recursion depth by
increasing the prologue size or complexity of function body to other
optimizers.

When profile feedback is available (see -fprofile-generate) the actual
recursion depth can be guessed from probability that function will recurse via
given call expression. This parameter limits inlining only to call expression
whose probability exceeds given threshold (in percents). The default value is
10.

early-inlining-insns

Specify growth that early inliner can make. In effect it increases amount of
inlining for code having large abstraction penalty. The default value is 10.

max-early-inliner-iterations

max-early-inliner-iterations

Limit of iterations of early inliner. This basically bounds number of nested
indirect calls early inliner can resolve. Deeper chains are still handled by
late inlining.

comdat-sharing-probability

comdat-sharing-probability

Probability (in percent) that C++ inline function with comdat visibility
will be shared across multiple compilation units. The default value is 20.

min-vect-loop-bound

The minimum number of iterations under which a loop will not get vectorized
when -ftree-vectorize is used. The number of iterations after
vectorization needs to be greater than the value specified by this option
to allow vectorization. The default value is 0.

gcse-cost-distance-ratio

Scaling factor in calculation of maximum distance an expression
can be moved by GCSE optimizations. This is currently supported only in the
code hoisting pass. The bigger the ratio, the more aggressive code hoisting
will be with simple expressions, i.e., the expressions which have cost
less than gcse-unrestricted-cost. Specifying 0 will disable
hoisting of simple expressions. The default value is 10.

gcse-unrestricted-cost

Cost, roughly measured as the cost of a single typical machine
instruction, at which GCSE optimizations will not constrain
the distance an expression can travel. This is currently
supported only in the code hoisting pass. The lesser the cost,
the more aggressive code hoisting will be. Specifying 0 will
allow all expressions to travel unrestricted distances.
The default value is 3.

max-hoist-depth

The depth of search in the dominator tree for expressions to hoist.
This is used to avoid quadratic behavior in hoisting algorithm.
The value of 0 will avoid limiting the search, but may slow down compilation
of huge functions. The default value is 30.

max-unrolled-insns

The maximum number of instructions that a loop should have if that loop
is unrolled, and if the loop is unrolled, it determines how many times
the loop code is unrolled.

max-average-unrolled-insns

The maximum number of instructions biased by probabilities of their execution
that a loop should have if that loop is unrolled, and if the loop is unrolled,
it determines how many times the loop code is unrolled.

max-unroll-times

The maximum number of unrollings of a single loop.

max-peeled-insns

The maximum number of instructions that a loop should have if that loop
is peeled, and if the loop is peeled, it determines how many times
the loop code is peeled.

max-peel-times

The maximum number of peelings of a single loop.

max-completely-peeled-insns

The maximum number of insns of a completely peeled loop.

max-completely-peel-times

The maximum number of iterations of a loop to be suitable for complete peeling.

max-completely-peel-loop-nest-depth

The maximum depth of a loop nest suitable for complete peeling.

max-unswitch-insns

The maximum number of insns of an unswitched loop.

max-unswitch-level

The maximum number of branches unswitched in a single loop.

lim-expensive

The minimum cost of an expensive expression in the loop invariant motion.

iv-consider-all-candidates-bound

Bound on number of candidates for induction variables below that
all candidates are considered for each use in induction variable
optimizations. Only the most relevant candidates are considered
if there are more candidates, to avoid quadratic time complexity.

iv-max-considered-uses

The induction variable optimizations give up on loops that contain more
induction variable uses.

iv-always-prune-cand-set-bound

If number of candidates in the set is smaller than this value,
we always try to remove unnecessary ivs from the set during its
optimization when a new iv is added to the set.

scev-max-expr-size

Bound on size of expressions used in the scalar evolutions analyzer.
Large expressions slow the analyzer.

scev-max-expr-complexity

Bound on the complexity of the expressions in the scalar evolutions analyzer.
Complex expressions slow the analyzer.

omega-max-vars

The maximum number of variables in an Omega constraint system.
The default value is 128.

omega-max-geqs

The maximum number of inequalities in an Omega constraint system.
The default value is 256.

omega-max-eqs

The maximum number of equalities in an Omega constraint system.
The default value is 128.

omega-max-wild-cards

The maximum number of wildcard variables that the Omega solver will
be able to insert. The default value is 18.

omega-hash-table-size

The size of the hash table in the Omega solver. The default value is
550.

omega-max-keys

The maximal number of keys used by the Omega solver. The default
value is 500.

omega-eliminate-redundant-constraints

When set to 1, use expensive methods to eliminate all redundant
constraints. The default value is 0.

vect-max-version-for-alignment-checks

The maximum number of runtime checks that can be performed when
doing loop versioning for alignment in the vectorizer. See option
ftree-vect-loop-version for more information.

vect-max-version-for-alias-checks

The maximum number of runtime checks that can be performed when
doing loop versioning for alias in the vectorizer. See option
ftree-vect-loop-version for more information.

max-iterations-to-track

The maximum number of iterations of a loop the brute force algorithm
for analysis of # of iterations of the loop tries to evaluate.

hot-bb-count-fraction

Select fraction of the maximal count of repetitions of basic block in program
given basic block needs to have to be considered hot.

hot-bb-frequency-fraction

Select fraction of the entry block frequency of executions of basic block in
function given basic block needs to have to be considered hot

max-predicted-iterations

The maximum number of loop iterations we predict statically. This is useful
in cases where function contain single loop with known bound and other loop
with unknown. We predict the known number of iterations correctly, while
the unknown number of iterations average to roughly 10. This means that the
loop without bounds would appear artificially cold relative to the other one.

align-threshold

Select fraction of the maximal frequency of executions of basic block in
function given basic block will get aligned.

align-loop-iterations

A loop expected to iterate at lest the selected number of iterations will get
aligned.

tracer-dynamic-coverage

tracer-dynamic-coverage-feedback

This value is used to limit superblock formation once the given percentage of
executed instructions is covered. This limits unnecessary code size
expansion.

The tracer-dynamic-coverage-feedback is used only when profile
feedback is available. The real profiles (as opposed to statically estimated
ones) are much less balanced allowing the threshold to be larger value.

tracer-max-code-growth

Stop tail duplication once code growth has reached given percentage. This is
rather hokey argument, as most of the duplicates will be eliminated later in
cross jumping, so it may be set to much higher values than is the desired code
growth.

tracer-min-branch-ratio

Stop reverse growth when the reverse probability of best edge is less than this
threshold (in percent).

tracer-min-branch-ratio

tracer-min-branch-ratio-feedback

Stop forward growth if the best edge do have probability lower than this
threshold.

Similarly to tracer-dynamic-coverage two values are present, one for
compilation for profile feedback and one for compilation without. The value
for compilation with profile feedback needs to be more conservative (higher) in
order to make tracer effective.

max-cse-path-length

Maximum number of basic blocks on path that cse considers. The default is 10.

max-cse-insns

The maximum instructions CSE process before flushing. The default is 1000.

ggc-min-expand

GCC uses a garbage collector to manage its own memory allocation. This
parameter specifies the minimum percentage by which the garbage
collector's heap should be allowed to expand between collections.
Tuning this may improve compilation speed; it has no effect on code
generation.

The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
RAM >= 1GB. If getrlimit is available, the notion of "RAM" is
the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If
GCC is not able to calculate RAM on a particular platform, the lower
bound of 30% is used. Setting this parameter and
ggc-min-heapsize to zero causes a full collection to occur at
every opportunity. This is extremely slow, but can be useful for
debugging.

ggc-min-heapsize

Minimum size of the garbage collector's heap before it begins bothering
to collect garbage. The first collection occurs after the heap expands
by ggc-min-expand% beyond ggc-min-heapsize. Again,
tuning this may improve compilation speed, and has no effect on code
generation.

The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which
tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but
with a lower bound of 4096 (four megabytes) and an upper bound of
131072 (128 megabytes). If GCC is not able to calculate RAM on a
particular platform, the lower bound is used. Setting this parameter
very large effectively disables garbage collection. Setting this
parameter and ggc-min-expand to zero causes a full collection
to occur at every opportunity.

max-reload-search-insns

The maximum number of instruction reload should look backward for equivalent
register. Increasing values mean more aggressive optimization, making the
compile time increase with probably slightly better performance. The default
value is 100.

max-cselib-memory-locations

The maximum number of memory locations cselib should take into account.
Increasing values mean more aggressive optimization, making the compile time
increase with probably slightly better performance. The default value is 500.

reorder-blocks-duplicate

reorder-blocks-duplicate-feedback

Used by basic block reordering pass to decide whether to use unconditional
branch or duplicate the code on its destination. Code is duplicated when its
estimated size is smaller than this value multiplied by the estimated size of
unconditional jump in the hot spots of the program.

The reorder-block-duplicate-feedback is used only when profile
feedback is available and may be set to higher values than
reorder-block-duplicate since information about the hot spots is more
accurate.

max-sched-ready-insns

The maximum number of instructions ready to be issued the scheduler should
consider at any given time during the first scheduling pass. Increasing
values mean more thorough searches, making the compilation time increase
with probably little benefit. The default value is 100.

max-sched-region-blocks

The maximum number of blocks in a region to be considered for
interblock scheduling. The default value is 10.

max-pipeline-region-blocks

The maximum number of blocks in a region to be considered for
pipelining in the selective scheduler. The default value is 15.

max-sched-region-insns

The maximum number of insns in a region to be considered for
interblock scheduling. The default value is 100.

max-pipeline-region-insns

The maximum number of insns in a region to be considered for
pipelining in the selective scheduler. The default value is 200.

min-spec-prob

The minimum probability (in percents) of reaching a source block
for interblock speculative scheduling. The default value is 40.

max-sched-extend-regions-iters

The maximum number of iterations through CFG to extend regions.
0 - disable region extension,
N - do at most N iterations.
The default value is 0.

max-sched-insn-conflict-delay

The maximum conflict delay for an insn to be considered for speculative motion.
The default value is 3.

sched-spec-prob-cutoff

The minimal probability of speculation success (in percents), so that
speculative insn will be scheduled.
The default value is 40.

The maximum size of the lookahead window of selective scheduling. It is a
depth of search for available instructions.
The default value is 50.

selsched-max-sched-times

The maximum number of times that an instruction will be scheduled during
selective scheduling. This is the limit on the number of iterations
through which the instruction may be pipelined. The default value is 2.

selsched-max-insns-to-rename

The maximum number of best instructions in the ready list that are considered
for renaming in the selective scheduler. The default value is 2.

max-last-value-rtl

The maximum size measured as number of RTLs that can be recorded in an expression
in combiner for a pseudo register as last known value of that register. The default
is 10000.

integer-share-limit

Small integer constants can use a shared data structure, reducing the
compiler's memory usage and increasing its speed. This sets the maximum
value of a shared integer constant. The default value is 256.

min-virtual-mappings

Specifies the minimum number of virtual mappings in the incremental
SSA updater that should be registered to trigger the virtual mappings
heuristic defined by virtual-mappings-ratio. The default value is
100.

virtual-mappings-ratio

If the number of virtual mappings is virtual-mappings-ratio bigger
than the number of virtual symbols to be updated, then the incremental
SSA updater switches to a full update for those symbols. The default
ratio is 3.

ssp-buffer-size

The minimum size of buffers (i.e. arrays) that will receive stack smashing
protection when -fstack-protection is used.

max-jump-thread-duplication-stmts

Maximum number of statements allowed in a block that needs to be
duplicated when threading jumps.

max-fields-for-field-sensitive

Maximum number of fields in a structure we will treat in
a field sensitive manner during pointer analysis. The default is zero
for -O0, and -O1 and 100 for -Os, -O2, and -O3.

prefetch-latency

Estimate on average number of instructions that are executed before
prefetch finishes. The distance we prefetch ahead is proportional
to this constant. Increasing this number may also lead to less
streams being prefetched (see simultaneous-prefetches).

simultaneous-prefetches

Maximum number of prefetches that can run at the same time.

l1-cache-line-size

The size of cache line in L1 cache, in bytes.

l1-cache-size

The size of L1 cache, in kilobytes.

l2-cache-size

The size of L2 cache, in kilobytes.

min-insn-to-prefetch-ratio

The minimum ratio between the number of instructions and the
number of prefetches to enable prefetching in a loop.

prefetch-min-insn-to-mem-ratio

The minimum ratio between the number of instructions and the
number of memory references to enable prefetching in a loop.

use-canonical-types

Whether the compiler should use the “canonical” type system. By
default, this should always be 1, which uses a more efficient internal
mechanism for comparing types in C++ and Objective-C++. However, if
bugs in the canonical type system are causing compilation failures,
set this value to 0 to disable canonical types.

switch-conversion-max-branch-ratio

Switch initialization conversion will refuse to create arrays that are
bigger than switch-conversion-max-branch-ratio times the number of
branches in the switch.

max-partial-antic-length

Maximum length of the partial antic set computed during the tree
partial redundancy elimination optimization (-ftree-pre) when
optimizing at -O3 and above. For some sorts of source code
the enhanced partial redundancy elimination optimization can run away,
consuming all of the memory available on the host machine. This
parameter sets a limit on the length of the sets that are computed,
which prevents the runaway behavior. Setting a value of 0 for
this parameter will allow an unlimited set length.

sccvn-max-scc-size

Maximum size of a strongly connected component (SCC) during SCCVN
processing. If this limit is hit, SCCVN processing for the whole
function will not be done and optimizations depending on it will
be disabled. The default maximum SCC size is 10000.

ira-max-loops-num

IRA uses a regional register allocation by default. If a function
contains loops more than number given by the parameter, only at most
given number of the most frequently executed loops will form regions
for the regional register allocation. The default value of the
parameter is 100.

ira-max-conflict-table-size

Although IRA uses a sophisticated algorithm of compression conflict
table, the table can be still big for huge functions. If the conflict
table for a function could be more than size in MB given by the
parameter, the conflict table is not built and faster, simpler, and
lower quality register allocation algorithm will be used. The
algorithm do not use pseudo-register conflicts. The default value of
the parameter is 2000.

ira-loop-reserved-regs

IRA can be used to evaluate more accurate register pressure in loops
for decision to move loop invariants (see -O3). The number
of available registers reserved for some other purposes is described
by this parameter. The default value of the parameter is 2 which is
minimal number of registers needed for execution of typical
instruction. This value is the best found from numerous experiments.

loop-invariant-max-bbs-in-loop

Loop invariant motion can be very expensive, both in compile time and
in amount of needed compile time memory, with very large loops. Loops
with more basic blocks than this parameter won't have loop invariant
motion optimization performed on them. The default value of the
parameter is 1000 for -O1 and 10000 for -O2 and above.

max-vartrack-size

Sets a maximum number of hash table slots to use during variable
tracking dataflow analysis of any function. If this limit is exceeded
with variable tracking at assignments enabled, analysis for that
function is retried without it, after removing all debug insns from
the function. If the limit is exceeded even without debug insns, var
tracking analysis is completely disabled for the function. Setting
the parameter to zero makes it unlimited.

min-nondebug-insn-uid

Use uids starting at this parameter for nondebug insns. The range below
the parameter is reserved exclusively for debug insns created by
-fvar-tracking-assignments, but debug insns may get
(non-overlapping) uids above it if the reserved range is exhausted.

ipa-sra-ptr-growth-factor

IPA-SRA will replace a pointer to an aggregate with one or more new
parameters only when their cumulative size is less or equal to
ipa-sra-ptr-growth-factor times the size of the original
pointer parameter.

graphite-max-nb-scop-params

To avoid exponential effects in the Graphite loop transforms, the
number of parameters in a Static Control Part (SCoP) is bounded. The
default value is 10 parameters. A variable whose value is unknown at
compile time and defined outside a SCoP is a parameter of the SCoP.

graphite-max-bbs-per-function

To avoid exponential effects in the detection of SCoPs, the size of
the functions analyzed by Graphite is bounded. The default value is
100 basic blocks.

loop-block-tile-size

Loop blocking or strip mining transforms, enabled with
-floop-block or -floop-strip-mine, strip mine each
loop in the loop nest by a given number of iterations. The strip
length can be changed using the loop-block-tile-size
parameter. The default value is 51 iterations.

devirt-type-list-size

IPA-CP attempts to track all possible types passed to a function's
parameter in order to perform devirtualization.
devirt-type-list-size is the maximum number of types it
stores per a single formal parameter of a function.

lto-partitions

Specify desired number of partitions produced during WHOPR compilation.
The number of partitions should exceed the number of CPUs used for compilation.
The default value is 32.

lto-minpartition

Size of minimal partition for WHOPR (in estimated instructions).
This prevents expenses of splitting very small programs into too many
partitions.

cxx-max-namespaces-for-diagnostic-help

The maximum number of namespaces to consult for suggestions when C++
name lookup fails for an identifier. The default is 1000.